This invention relates to a method and apparatus for obtaining an air gap flux signal for an AC machine and, more specifically, to a method and apparatus for obtaining an air gap flux signal for an AC machine directly from terminal voltage applied to, and line current drawn by the machine.
In many applications where feedback control of an AC machine is desired, it is often useful to have some indication of the phase angle difference (.theta.) between machine stator current and air gap flux. Such phase angle measurement typically is achieved by first deriving signals representing the air gap flux in each machine phase and then combining the air gap flux components with preselected components of stator current to yield a signal proportional to sin (.theta.). Having an air gap flux signal which is substantially sinusoidal and free from notches and spikes is particularly useful when the AC machine is powered from a DC source by an inverter because then the phase angle signal sin (.theta.) can be used to generate synchronizing information to control inverter thyristor firing intervals, thus synchronizing the inverter to the machine.
In the past, various approaches have been disclosed for developing an air gap flux signal for an AC machine. One such method disclosed in U.S. Pat. No. 3,855,529, issued June 14, 1971 to Langweiler teaches the placement of a measuring detector extending in the circumferential direction of the stator for determination of air gap flux. The detector is disclosed as being a Hall generator or group of Hall generators.
A major disadvantage with measurement of flux in this manner is that it requires modification of the motor to accommodate the detector. This requires additional expense and eliminates the use of off the shelf motors.
Another method for obtaining an air gap flux signal is disclosed in U.S. Pat. No. 3,909,687 issued Sept. 30, 1975 to Abbondanti and assigned to Westinghouse Electric Corporation. Abbondanti teaches the measurement of flux by generating a signal representative of the reactive power, W.sub.X in each phase of the machine. The reactive power W.sub.X is proportional to the square of the magnetization current I.sub.M which is, in turn, proportional to air gap flux.
The difficulty with the development of an air gap flux signal as disclosed in this manner is that it is dependent on the magnetization current I.sub.M of the machine. The magnetization current, I.sub.M, is in turn, dependent on the machine construction and materials used therein. Thus, it is not inconceivable, owing to variations in the ferromagnetic properties of steel used in such motor construction, for I.sub.M to vary as much as 20% to 30% between two identically constructed motors. Hence, the flux signal may also vary in this amount.
Still another approach disclosed for developing an air gap flux signal within AC machines is taught in U.S. Pat. No. 3,593,083 issued July 13, 1971 to Blaschke. In the Blaschke disclosure, appropriate values of the line voltage, the line current, and the differential portion of the line current are combined and integrated to produce a signal proportional to machine air gap flux.
The major difficulty with the developement of an air gap flux signal from a combination of signals representing line current and the differential portion of line current becomes apparent if the line current signal waveform is not a smooth function. This often occurs when the machine is driven from a DC source by an inverter causing a current waveform having notches and spikes. Differentiation of the current as taught by Blaschke may lead to a discontinuous function thus resulting in an erroneous air gap flux signal.
The present invention alleviates these difficulties by providing an air gap flux signal for an AC machine directly from AC terminal voltages and line currents which signal is substantially smooth, continuous and sinusoidal notwithstanding variation in machine construction.